On some Python builds, Unicode characters above U+FFFF get escaped
to two question marks ?? instead of just one ?. This is not a real
problem, but for testing it is a nuisance when the results depend
on the platform.
This class had been added in the `morx` branch, and I wanted
to merge it into master. While writing tests for it, I noticed
that `otConverters` has meanwhile been extended by an `UInt8`
converter. Therefore, only adding a test for the existing
implementation.
I've got some changes in the `morx` branch which I'd like to merge
into master. So I'm writing this unit test as an example how to
test `otConverter` code.
